Presently, position locating systems may include a portable remote unit which comprises a receiver for receiving signals from a satellite positioning system (e.g., a global positioning system (GPS)), telecommunications circuitry, clock circuitry and a microprocessor for analyzing coded signals received from the satellite positioning system and from a telecommunications link, for example, a wireless communication network. The microprocessor of this integrated positioning and telecommunications system may analyze the coded signals to determine the latitude and longitude of the portable remote unit. An exemplary integrated positioning and telecommunications system can be found in, for example, U.S. Pat. No. 5,043,736, issued Aug. 27, 1991, entitled "Cellular Position Locating System", by Darnell et al., which discloses a portable locating unit useful both as a cellular telephone and a portable global positioning system.
The determination of the position of the remote unit may be calculated with, or without, information provided to the remote unit over the telecommunications link, e.g., over the wireless communication network.
In one type of a GPS system, which operates without information provided over the telecommunications link, the precise position of each of a plurality of satellites at any given time are transmitted to the Earth in the form of coded signals. The coded signals contain information as to the position of each of the satellites in space with respect to GPS time, and also, an indication of the precise time at which each one of the signals was transmitted from each one of the satellites. This information may be referred to as ephemeris data. The coded signals also include high rate repetitive signals referred to as pseudorandom (PN) sequences. Each GPS satellite broadcasts a signal with a unique offset to a PN sequence.
The coded signals are transmitted by the GPS satellites within a predetermined band of frequencies. The remote units search the predetermined band of frequencies in an effort to acquire the coded signals from in-view GPS satellites. In the acquisition process, a remote unit receives signals from the GPS satellites and performs a correlation operation. The correlation operation evaluates the received signals and searches for known PN sequences. The detection of a known PN sequence within a received signal is an indication that the received signal is a GPS coded signal. Once the PN sequence search is complete and a coded signal is acquired, the range or distance between the GPS satellite transmitting the signal and a receiver, e.g. the remote unit, may be determined. The distance is determined by using the time of transmission of the coded signal and by noting the time that the signal is received at the remote unit from one of the satellites. The remote unit calculates a propagation time delay from the time difference, i.e. from the difference between the time the coded signal was transmitted and the time the signal was received at the remote unit. The calculated time delay, when multiplied by the speed of propagation of the signal, provides the range or distance value, referred to as a "pseudorange", between the transmitting satellite and the remote unit. The distance is referred to as a "pseudorange" because, while the actual calculation may be accurate, errors may be introduced in the data by the fact, e.g., that a local clock, which is generated in the remote unit, may not be precisely synchronized with GPS time, and also by the fact that signal propagation through the atmosphere may encounter delays.
Once the pseudorange computations are completed, the position of the remote unit is determined by using the pseudoranges and the satellite timing and ephemeris data. Typically, GPS signals from at least two or three line-of-sight positioning satellites are needed to supply sufficient information to derive accurate position determinations at an Earth-based station, such as the remote unit. Conventionally four such satellites are used to determine a terrestrial position estimate, three for triangulation and one for correcting for clock bias.
In another type of a GPS system, the position of the remote unit is determined by utilizing the positioning signals received from in-view satellites (as described above) and also satellite information received over a telecommunications link from a base station. The telecommunications link may be, for example, a two-way page system or a cellular communication system. Accordingly, the remote unit utilizes the information received from the GPS satellites and from the telecommunications link to compute pseudoranges and, in turn, to compute its latitudinal and longitudinal position. An exemplary embodiment of a position locating systems which utilizes positioning signals received from in-view satellites and satellite information received over a data communication link can be found in U.S. Pat. No. 5,663,734, issued Sep. 2, 1997, entitled "GPS Receiver and Method For Processing GPS Signals", by Norman F. Krasner. Similar systems are also disclosed in U.S. Pat. No. 5,225,842, issued Jul. 6, 1993, entitled "Vehicle Tracking System Employing Global Positioning System (GPS) Satellites", by Brown et al. and in U.S. Pat. No. 5,365,450, issued Nov. 15, 1994, entitled "Hybrid GPS/Data Line Unit For Rapid, Precise, and Robust Position Determination", by Schuchman et al.
In Schuchman et al., for example, there is disclosed a specialized server which resides on a cellular phone system. The specialized server includes a satellite almanac database which contains ephemeris and time models of a GPS satellite constellation. In one embodiment, the ephemerides and time model information is provided to the GPS receiver for use in a search and acquisition mode.
In FIG. 1, a conventional GPS locating system 10 is shown. A remote unit 12 of the GPS locating system 10 searches a predetermined band of frequencies to acquire coded signals 14 transmitted by orbiting GPS satellites 16. The remote unit 12 also establishes communication, via data link 18, with a service center 20 which services a region of a cellular phone system 22. The cellular phone system 22 includes a satellite information database 24 which contains ephemerides and timing data for the GPS satellites 16 within a GPS satellite constellation.
The remote unit 12 of this conventional GPS locating system 10 uses the coded signals 14 and ephemeris and timing data to determine the latitudinal and longitudinal position of the remote unit 12. The remote unit 12 also uses the data from the satellite information database 24 to aid its search for in-view GPS satellites 16 of the GPS constellation. A prior knowledge of the location of the GPS satellites 16 can decrease the time required to acquire the coded signals 14 from the GPS satellites 16.
The satellite information database 24 is stored in a specialized server 26 of the cellular phone system 22. Thus, the ephemerides and timing data of the GPS satellites 16 can be requested by the remote unit 12, retrieved from the satellite information database 24, and passed by the cellular phone system 22 through the service center 20 over data link 18 to the requesting remote unit 12. The received ephemerides and timing data can then be used, with the received coded signals 14, to determine the latitude and longitude of the remote unit 12.
As can be appreciated, delays may be experienced in this process of requesting and receiving data from the satellite information data base 24 of the cellular phone system 22. For example, delays in the transmission of a request for data by the remote unit 12, the reception and processing of the request in the cellular phone system 22, and the retrieval and transmission of data from the satellite information data base 24 back to the remote unit 12, may compromise the validity of the information provided. That is, the delays may result in the remote unit 12 receiving information that a GPS satellite is at a first position when, in fact, the GPS satellite has passed the first position and is in a second position. Under these circumstances the attempt to aid the search for in-view satellites has failed and may have increased the actual acquisition time.
As demonstrated above, the ability of a remote station to rapidly locate, track and acquire position information from in-view, earth orbiting GPS satellites is desirable. Satellite ephemerides and timing information can be provided to aid the acquisition process. While it is also desirable to achieve the goal of rapid acquisition and tracking, it is undesirable to provide a specialized server in a radiotelephone system to accomplish the goal.